1. Field of the Invention
This invention relates to an electronic endoscope for displaying a body-cavity image on a TV monitor, which comprises a video-scope having an image sensor and a video-processor. In particular, this invention relates to a control of a brightness of an object image.
2. Description of the Related Art
In the electronic endoscope, the video-scope includes an optical light guide extended therethrough, which is formed as a bundle of optical fibers. On the other hand, the video-processor includes a light source, such as a halogen lamp. When the video-scope is connected to the video-processor, a proximal end of the optical light guide is optically connected to the light source. Thus, an object to be photographed is illuminated by light radiating from a distal end of the optical light guide, and an object image is formed on the image sensor provided at the distal end of the video-scope. Then, the object image, formed on the image sensor, is converted into analog image-pixel signals by photoelectric conversion.
The analog image-pixel signals are fed to the video-processor and are suitably processed, so that video signals are generated., The video signals are then output to a TV monitor. Thus, an body-cavity image (for example, a stomach image) is displayed on the monitor.
In general, to maintain a proper brightness of the object image displayed on the monitor, a quantity of light radiating from the distal end of the optical light guide should be regulated. Conventionally, for a regulation of the quantity of light, a stop (diaphragm) provided between the light source and the proximal end of the light guide is controlled, such that the brightness of the object image is maintained at a constant level.
In this case, luminance signals are generated from one frame's worth of the analog image-pixel signals, which are successively extracted from the image sensor, and an average luminance value is successively calculated on the basis of the luminance signals. Then, the stop is controlled on the basis of the difference between the average luminance value and a predetermined reference value. According to the image-pixel signals successively read from the image sensor, the regulation of the quantity of light is successively performed i.e. at regular time-intervals.
In this way, by regulating the quantity of light automatically, the brightness of the object image is maintained at a proper level. Thus, on the monitor, the object image is always displayed with a proper brightness
However, in such an automatic regulation of the quantity of light, occasionally the average luminance value can not be calculated. For example, when the average luminance value is calculated by a histogram processing, an imperfect histogram-data, that is, a histogram-data not corresponding to one frame's worth of the object image is occasionally generated. In this case, as the average luminance value is not calculated, the stop is controlled on the basis of a preceding average luminance value, which is calculated from the preceding calculation.
However, the brightness of the object image formed on the image sensor has been already corrected, and the stop is driven in error, so that a hunting-situation, in which the stop does not converge to a proper position for a long time, occurs.
On the other hand, if the regulation of the quantity of light is performed at longer time-intervals in order to solve the above problems, the stop can not be rapidly controlled when the brightness of the object image changes considerably.